1. Field of the Invention
The present invention relates to a protective material capable of protecting a reforming catalyst and component materials from an electrolyte, in a molten carbonate fuel cell for generating electric energy by consuming hydrogen, which is formed from a raw fuel such as hydrocarbons.
2. Description of the Prior Art
FIG. 1 shows a perspective view of a major part of a conventional internal reforming type molten carbonate fuel cell disclosed in, for instance, Japanese Patent Application Laid-Open (KOKAI) No. 60-32255 (1985). In the figure, 1 denotes an electrolyte matrix composed of a porous ceramic with spaces therein filled with a carbonate used as an electrolyte, 2 denotes a fuel electrode (anode) composed of porous nickel or the like, and 3 denotes an oxidizing agent electrode (cathode) composed of a porous material such as nickel oxide. The fuel electrode 2 and the oxidizing agent electrode 3 are disposed opposite to each other, with the electrolyte matrix 1 therebetween, and these constitute a single unit of a cell. In the figure, 4 denotes an oxidizing agent passage provided for the oxidizing agent electrode 3, 5 denotes a perforated fuel-side spacer disposed in contact with the fuel electrode 2, and 6 denotes a rib provided perpendicularly to the fuel-side spacer 5, with the spacer 5 and the ribs 6 defining fuel gas passages 7. By 8 is denoted a fuel reforming catalyst packed in the fuel gas passages 7.
FIG. 2 shows a system diagram illustrating the construction of a fuel cell power generation system employing an external reforming type molten carbonate fuel cell body disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 60-230365. In the diagram, a fuel treating device 9 and an air supply device 10 are connected to a molten carbonate fuel cell body 11. A combustor 12 is provided for oxidizing a fuel gas not reacted in the fuel cell body 11, a heat exchanger 13 is provided for removing surplus heat generated in the fuel cell body 11 to the exterior of the system, and a circulating blower 14 is provided for circulating an oxidized gas, which serves as a coolant.
Operations of the fuel cells will now be explained. In the internal reforming type fuel cell, the fuel reforming catalyst 8 provided in the fuel gas passages 7 adjacent to the fuel electrode 2 is used to induce a reforming reaction of a hydrocarbon or alcohol contained in the fuel gas, thereby forming hydrogen. Hydrogen thus formed through the reforming reaction in the fuel gas passages 7 is oxidized to water by an electrochemical reaction at the fuel electrode 2 adjacent to the passages 7. Part of the energy generated on the oxidation is converted to electric energy. For a long, stable operation of the internal reforming type fuel cell, therefore, it is essential to maintain a stable activity of the reforming catalyst 8 for a long time. However, in the conventional internal reforming type fuel cell, the reforming catalyst 8 is held adjacent to the fuel electrode 2 and, therefore, adhesion of the electrolyte to the reforming catalyst 8 is inevitable. On the other hand, the external reforming type fuel cell power generation system operates as follows. In a conventional fuel cell power generation system as shown in FIG. 2, the electrolyte or a substance formed therefrom which is contained in the fuel gas or oxidized gas discharged from the molten carbonate fuel cell body 11 is fed, as it is, downstream through the power generation system. This causes corrosion of component members of piping or apparatus, or lowering in the activity of the reforming catalyst. At a low-temperature part, in particular, problems such as solidification of the electrolyte and clogging of passages are generated. Thus, the conventional fuel cell power generation system has generally had the problem of reduction of the characteristics of the component apparatuses and the problem of a short service life.
Namely, in the conventional molten carbonate fuel cell power generation system as mentioned above, whether the molten carbonate fuel cell body is of the internal reforming type or of the external reforming type, the electrolyte evaporated or spattered into the reaction gas in the fuel cell body or the substance formed from the electrolyte will adhere to the materials constituting the fuel cell power generation system inclusive of the fuel cell body itself, to impair the characteristics of the constituent materials, thereby making it difficult to operate the system for a long time.
More particularly, for instance, the internal reforming type fuel cell body has the following problem. The reforming catalyst 8 is reduced in performance when contacted by the electrolyte held in the electrolyte matrix 1 or by decomposition products of the electrolyte. This arises from the migration of the electrolyte, through evaporation or the like, from the matrix 1 to the place where the reforming catalyst 8 is disposed. The degradation of the performance of the reforming catalyst 8, for instance, a nickel catalyst takes place as follows. When nickel particles (about 200 .ANG. in diameter) makes contact with the electrolyte, nickel dissolves in the electrolyte as NiO, and growth of particles occurs. The particles grow to a particle size of about 500 to 3000 .ANG., with a reduction in surface area. Alternatively, the adhered electrolyte lowers the catalytic activity of the catalytically active substance (nickel particles). Thus, the performance of the catalyst is degraded. The dissolution of NiO in the electrolyte is discussed in literature [C. E. Baumgartner, "Solubility and Transport of NiO Cathodes in Molten Carbonate Fuel Cells", J. of American Ceramic Society, Vol. 69 (1986), pp. 162-168].
On the other hand, the use of the external reforming type fuel cell body has the following problems. First, the vapor of the electrolyte reacts with a main constituent material of piping or apparatus, for example, stainless steel parts at high-temperature, to form a corroded layer, thereby enbrittling or weakening the constituent material. In addition, adhesion or solidification of the electrolyte occurs at low-temperature parts to cause clogging, especially at narrow parts. Besides, as a total effect of the above, the electrolyte causes reduction in the characteristics and service life of the circulating blower 14, the heat exchanger 13, etc. For example, in a single-cell test on molten carbonate fuel cells, adhesion or solidification of the electrolyte was observed at narrow parts, particularly at an outlet pipe on the fuel gas side. For a steady operation, it was necessary to clean the piping, for instance, at a time interval of 1000 to 3000 hours.